scpi: document callers' responsibility to free getters' allocated memory

scpi: document callers' responsibility to free getters' allocated memory

Some of the SCPI getter routines allocate memory for response data which
callers need to free after use. Document this responsibility in more
places, and rephrase existing locations for consistency. See a word-diff
for the essence of the commit.

It's assumed that the separate paragraph in the function description is
better perceived than something that's hidden in the parameter or return
value discussion. Also reference the corresponding free routine by name
for callers' convenience (data types will differ among routines). Some
free routines can be called unconditionally, others require NULL checks
at the caller's.

Reported-By: Ralf <redacted>